Field and Laboratory Analysis of Pyroclastic Deposits for Undergraduate Volcanology Courses

Jeffrey Templeton, Western Oregon University

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This activity consists of a series of linked field and laboratory exercises designed for undergraduate volcanology courses. The activity incorporates several interconnected exercises that employ field observations, data collection, and granulometric analyses to interpret pyroclastic deposits in central Oregon. Based on their interpretations, students then consider regional volcanic hazards through a reflective writing assignment. Given the modularity of this activity, different exercises can be conducted depending on the resources available to individual instructors. This activity engages students in the scientific process, through observation, data analysis, and interpretation. Students gain experience communicating results in written form to a diverse audience. Team work and collaboration are built into the activity through group field and laboratory work. This activity can be used for program assessment, as it dovetails with geoscience education initiatives, emphasizing broader scale objectives of preparing future geoscientists to solve challenging problems in the 21st century.

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This activity is designed for an upper-level volcanology course for undergraduates. The prerequisite for this elective course is a single lower-level introductory physical geology course, so the range of student backgrounds in the class includes Earth and Environmental Science majors, Environmental Studies minors, and Integrated Science Teacher Education majors.

Skills and concepts that students must have mastered

Students should be introduced to different types of pyroclastic deposits, as well as textural features and characteristics that are used to distinguish them in the field, prior to beginning this activity. Other skills and concepts can be covered on a need-to-know basis and learned in the context of engaging the various parts of the learning activity.

How the activity is situated in the course

This activity consists of a series of exercises beginning with a weekend field trip typically conducted in week 6 of a 10-week term. During class the week after the field trip, students are assigned the first part of the granulometric analysis exercise in which they sieve their samples and submit results the following week. The second part of the analysis assignment is assigned the following week. The writing assignment is typically assigned during week 9 with a due date during finals week.


Content/concepts goals for this activity

This learning activity engages students in the study of pyroclastic deposits in field and laboratory settings. Upon completion of this activity, students will be able to:
-Document textural characteristics of pyroclastic deposits in the field, including grain size, sorting, and grading, take measurements and record data, and collect samples for further analysis
-Utilize sieves to conduct granulometric analyses of pyroclastic samples
-Use grain size data to calculate parameters that are used to characterize pyroclastic deposits

Higher order thinking skills goals for this activity

This learning activity provides students with an opportunity to gain experience conducting authentic scientific investigations. Students are actively involved in the scientific process, through observation, data analysis, and interpretation. Higher order thinking skills include:
-Gathering field data, making observations, and formulating hypotheses for the origin of pyroclastic deposits
-Analyzing and interpreting granulometric data from pyroclastic deposits to test hypotheses

Other skills goals for this activity

This learning activity emphasizes two primary professional skills, including written communication and teamwork. Students are asked to communicate results in written form to a diverse audience, in this case educated non-scientists and the general public. Teamwork and collaboration are built into the learning activity through group field and laboratory work.

Description and Teaching Materials

This activity consists of a series of linked exercises that employ field observations, data collection, and granulometric analyses to interpret pyroclastic deposits in central Oregon. Given the modular nature of this activity, different exercises can be conducted depending on the resources available to individual instructors. Student worksheets, assignment handouts with detailed instructions, and assessment rubrics are provided as Supporting Materials.

The field exercise is conducted as part of a required weekend field trip to study volcanology in central Oregon. Students spend one half-day studying a volcaniclastic deposit associated with the Tumalo volcanic center (Hill and Taylor, 1990). The lower part of the deposit consists of the Bend Pumice and the upper part is the Tumalo Tuff. At this locality, the Bend Pumice and Tumalo Tuff (Hill and Taylor, 1990; Conrey, et al., 2004) are well exposed, allowing for excellent access through the vertical succession, with student groups spread out across the face of the outcrop (refer to Supporting Materials). The outcrop studied for this exercise is located on private land for which permission is necessary to gain access.

Upon arrival at the exposure, students are broken into teams and assigned a specific part of the section. Two student groups are assigned to the lower part of the deposit, one group is assigned to the thin fine-grained unit about 3 meters above the base of the deposit, and depending on the class size, 2 to 4 student groups are assigned sections in the upper part of the sequence, moving vertically upward. Working in groups and focusing on their assigned interval, students describe components and estimate percentages, measure pumice and lithic sizes, characterize sorting and grading, and collect a sample for further grain size analysis in the laboratory (refer to Supporting Materials). Each group also constructs a stratigraphic column for the entire deposit, with the field observations and data from their interval referenced to the column. After each group has completed the field tasks, the class gathers to share key highlights from their interval with the other groups, with the instructor guiding the class towards using their observations to make interpretations about the origin for each part of the section.

Upon returning from the field, students are assigned the next exercise in which they work in groups to sieve their sample and determine weights for each size fraction (refer to Supporting Materials). Students use a set of sieves at increments from -4 phi (16 mm) to 4 phi (0.0625 mm) to complete a granulometric analysis of their sample from the Bend Pumice/Tumalo Tuff exposure. To avoid breaking clasts, students are advised to shake the sieves gently by hand. Each size fraction is weighed and students report the grain size data in provided tables. After sieving, students are instructed to carefully clean the sieves with provided brushes, to wipe down the counters, and sweep the work area. This process typically takes student groups about two hours of out of class time to complete, and students are given a week to complete this part of the activity.

Each student group submits their grain-size data to the instructor upon completion of sieving. The instructor compiles this data into a single Excel file. At this point, instructors can choose to have students calculate weight percent and cumulative weight percent for each size fraction, or complete this for the students. Owing to time constraints, I typically do these calculations in Excel and provide the data set to the class for the next exercise in the sequence (refer to Supporting Materials). Using provided cumulative wt. % graph paper, students graph the grain size data for all of the class samples, determine select phi (ϕ) values, and calculate Inman (1952) parameters using published formulas to characterize sorting. Students also plot median diameter (Mdϕ) vs. graphical standard deviation (σϕ) and prepare histograms for the class data set. The graphs are then used, in concert with field observations, to make interpretations about the pyroclastic origins for the Bend Pumice and Tumalo Tuff (refer to Supporting Materials).

The culminating exercise is a writing assignment in which students, from the perspective of a U.S.G.S. volcanologist, prepare a mock scenario report for the Bend, Oregon City Council to address regional volcanic hazards. As described in the assignment materials provided to the students (refer to Supporting Materials), the council is concerned about volcanic hazards that the city may face from the recently rejuvenated Tumalo volcanic center located ~20 km west of Bend. About 300,000-400,000 years ago, the Bend Pumice and Tumalo Tuff were erupted from this silicic volcanic center (Sarna-Wojcicki et al., 1989; Hill and Taylor, 1990; Conrey, et al., 2004); study of these deposits is crucial in understanding how future eruptions may proceed. In this role, the student scientists have conducted preliminary field and laboratory work on a well-exposed section of the Bend Pumice/Tumalo Tuff, so the Bend City Council is particularly interested in getting this report for planning future housing developments in the region.

Supporting Materials:
Figures for Analysis of Pyroclastic Deposits Learning Activity (Acrobat (PDF) 256kB Sep1 23)
Describing Pyroclastic Deposits in the Field (Microsoft Word 2007 (.docx) 3MB Sep1 23)
Granulometric Analysis Laboratory Instructions and Data Worksheet (Microsoft Word 2007 (.docx) 20kB Sep11 23)
Granulometric Analysis Data Graphing and Interpretation (Microsoft Word 2007 (.docx) 866kB Sep11 23)
Pyroclastic Analysis Reflective Writing Assignment (Microsoft Word 2007 (.docx) 20kB Sep1 23)

Teaching Notes and Tips

Most of the methods for characterizing pyroclastic deposits in the field are fundamental to most field-based geologic exercises. To obtain maximum grain size for juvenile and lithic clasts, students are instructed to measure the 10 largest lithics and the 10 largest pumice clasts in 1-m2 areas from their assigned stratigraphic interval.

The methodology for conducting the granulometric analysis is based on procedures described in Appendix 1 of Cas and Wright (1987). While there are a variety of techniques for determining grain-size distributions of tephra (e.g., Buckland et al., 2021), this activity employs mechanical sieving because of its low cost and user-friendliness for undergraduate geoscience students (cf., Davies-Vollum, 2006; Videtich and Neal, 2012). Sieving involves separating the tephra into individual size fractions by passing the material through a series of nested sieves in which the aperture size decreases at one-phi (ϕ) intervals, where
ϕ= -log2⁡d
and d is the size of the sieve aperture in mm and measures the grain size assuming a spherical shape (Wentworth, 1922; Krumbein, 1934). Instructors will need a set of sieves ranging from -4 ϕ to 4 ϕ (16 mm to 0.0625 mm) (Table 1).

Table 1: List of sieves needed for granulometric analysis of pyroclastic materials.

U.S Standard Sieve Mesh # Millimeters (mm) Microns Phi (ϕ)
5/8" 16.0 -4.0
5/16" 8.0   -3.0
#5 4.0   -2.0
#10 2.0   -1.0
#18 1.0   0.0
#35 0.5 500 1.0
#60 0.25 250 2.0
#120 0.125 125 3.0
#230 0.0625 62.5 4.0

Students should be instructed to sieve by hand to avoid breakage of fragile tephra fragments (Walker, 1971). Instructors should also provide one or more digital scales for students to weigh each size fraction. It is recommended that at this stage of the activity, students submit their raw weight data to the instructor, which is used to calculate weight percentages for each size fraction and cumulative weight percent. The grain size distributions for the entire class data set are used to construct cumulative frequency curves with cumulative weight percent on the y-axis and particle size in phi units on the x-axis. Blank normal probability graph paper can be created using a graphing software package or sourced from the internet for student use. From these graphs, students determine Inman (1952) parameters for all class samples, including median diameter (Mdϕ), graphical standard deviation (σϕ), which is also a measure of sorting, and skewness (αϕ). Students should be provided with the following equations:
σϕ=(ϕ8416 )/2
αϕ=[(ϕ8416 )-Mdϕ ]/σϕ 
Following the classic paper of Walker (1971), students plot Mdϕ vs. σϕ for the entire class data set, as well as prepare histograms for each sample, all of which are used to interpret the pyroclastic origin of the deposits (Wohletz and Heiken, 1992).


The assessment strategy for the assignment uses a set of post-activity questions embedded into the final exam (refer to Supporting Materials). These questions require students to graph and analyze data from unknown pyroclastic deposits and interpret their origin. By splitting the questions into those pertaining to data analysis vs. interpretation, student skill levels can be assessed with respect to graphing, completing calculations, and analyzing data. Student learning gaps are identified with feedback on how to improve skill levels, for example making geologic interpretations from available data. Taking this a step further, student performance is also assessed using a modified version of the AACU Inquiry and Analysis Rubric ( that maps to institutional learning outcomes. The writing assignment is assessed using a scoring guide that is aligned with the requirements as outlined in the instructions (refer to Supporting Materials).

References and Resources

Buckland, H.M., Saxby, J., Roche, M., Meredith, P., Rust, A.C., Cashman, K.V., Engwell, S.L., 2021, Measuring the size of non-spherical particles and the implications for grain size analysis in volcanology: Jour. of Volcanology and Geothermal Research, v., 415, p. 1-22.
Cas, R.A.F., Wright, J. V., 1987, Volcanic Successions: Modern and Ancient: Allen and Unwin, London, 528 pp.
Conrey, R.M., Grunder, A.L., Schmidt, M.E., 2004, SOTA field trip guide-State of the Cascade Arc: stratocone persistence, mafic lava shields, and pyroclastic volcanism associated with intra-arc rift propagation, DOGAMI Open File Report O-04-04, 39 pp.
Davies-Vollum, K.S., 2006, Using grain size analysis as the basis for a research project in an undergraduate sedimentology course, Jour. of Geoscience Education, v. 54, p. 10-17.
Hill, B.E., Taylor, E.M., 1990, Oregon Central High Cascade pyroclastic units in vicinity of Bend, Oregon: Oregon Geology, v. 52, p. 125-126, 139-140.
Inman, D.L., 1952, Measures for describing the size distribution of sediments: Jour. of Sedimentary Research, v. 22, p. 125-145.
Krumbein, W.C., 1934, Size frequency distributions of sediments: Jour. of Sedimentary Research, v. 4, p. 65-77.
Sarna-Wojcicki, A.M., Meyer, C.E., Nakata, J.K., Scott, W.E., Hill, B.E., Slate, J.L., Russell, P.C., 1989, Age and correlation of mid-Quaternary ash beds and tuffs in the vicinity of Bend, Oregon, in Scott, W.E., Gardner, C.A., Sarna-Wojcicki, A.M., eds., Guidebook for field trip to the Mount Bachelor-South Sister-Bend area, central Oregon High Cascades: U.S. Geological Survey Open-File Report 89-965, p. 55-62.
Videtich, P.E., Neal, W.J., 2012, Using sieving and unknown sand samples for a sedimentation-stratigraphy class project with linkage to introductory courses: Jour. of Geoscience Education, v. 60, p. 325-336.
Walker, G.P.L., 1971. Grain-size characteristics of pyroclastic deposits. Jour. of Geology, v. 79, p. 696-714.
Wentworth, C.K., 1922, A scale of grade and class terms for clastic sediments. Jour. of Geology, v. 30, p. 377-392.
Wohletz, K., Heiken, G., 1992, Volcanlogy and Geothermal Energy: Berkeley, University of California Press; (accessed 24 August 2023).